It is known to limit the length of the outlet channels to a minimum, both for thermal reasons and in order to maintain a small pressure drop. The flow of the melt to the outlet channels takes place individually through conical sections in order to permit a uniform flow of the melt.
One disadvantage of this arrangement is that, because of the conical inlet sections, the number of outlet channels in the perforated plate is limited by the finite area of the inlet sections. The required stability of the perforated plate in the critical outlet region of the outlet channels which is subjected to thermal stresses also establishes technical limits.
U.S. Pat. No. 3,427,685 discloses a perforated plate in which, starting from a distributor channel, conically tapering inlet channels open into the outlet channels. The outlet channels are present only in limited number since each outlet channel is supplied by a separate inlet channel. The individual bores forming the outlet channels extend substantially over the entire width of the perforated plate and furthermore impair the quality of the product due to the pressure drop which is produced.
In another perforated plate construction shown in U.S. Pat. No. 3,857,665, pocket-shaped inlet channels are provided, each inlet channel opening into two flow channels of relatively short length. A disadvantage of this perforated plate construction is that disturbances in inflow occur in the region of the outlet channels at the bottom of the pocket, these disturbances being further increased by the formation of a bridge at the inlet pocket. When a large number of outlet channels is provided, the locally dense arrangement of adjacent outlet channels furthermore decreases the stability at the front surface of the perforated plate which is subjected to a high thermal load.
DE-OS 34 20 944 shows a perforated plate for the underwater granulating of plastics in which, for reducing the extrusion pressure and increasing the density of the outlet channels a primary hole of large diameter is provided from which several outlet channels extend parallel to the longitudinal axis of the hole. This construction has the disadvantage that the change in cross-section in the transfer region from the primary hole to the outlet channels results in a non-uniform feed of the melt, which impairs the quality of the product. The stability at the front surface of the perforated plate is also reduced by the provision of a plurality of outlet openings of the outlet channels which are separated from each other by only relatively thin and short webs.